A color line camera that is used, among others, for industrial real-time quality control, is composed of a color splitting prism, for instance of three linear CCD (Charge Coupled Device) cells, which thus include a line of single CCD elements, plus electronics. A CCD cell can include for example 1024 or 2048 light-sensitive CCD elements with a size that can vary for instance within the range 7–13 μm. A color splitting prism is a prism array compiled of several prism elements, and generally it separates the light emitted from the target through an objective, as well as the corresponding image information, in three partial colors, i.e. red, green and blue, so that partial images of different colors are created on different exit surfaces of the color splitting prism, or on image surfaces located in the vicinity of said exit surfaces. In order to make the CCD cells see the same spot in said three images, the CCD cells must be aligned accurately, at the precision of a few μm:s with respect to each other, and said alignment must be maintained both during the drying period of the glue and during the working period, when the cell is heated. There also exist surface-type CCD cells that include large numbers of CCD element rows, but the present invention concentrates in mounting linear CCD cells on the surface of a color splitting prism.
In the publications JP-4-290090 and WO-96/13930, there are used arrangements where the CCD cells are attached directly on the exit surfaces of the partial images of the color splitting prism by glue that is applied in between the prism and the cell. According to the publication WO-96/13930, there is used epoxy glue, in which case there arises the problem whether there is time to perform the alignment before the glue is hardened, as well as the thermal expansion coefficient of glue, which is different from that of glass. In the method according to the publication JP-4-290090, there is used a glue that is polymerized by means of ultraviolet light and heat, so that after applying the glue, the CCD cell is pressed on the prism surface and aligned, and the glue is hardened by irradiating with ultraviolet light. Here the problem is the thermal expansion coefficient of glue, which is different from that of glass. From the publication U.S. Pat. No. 4,323,918 there is known an arrangement where on the exit surfaces of the color splitting prism, there are first provided trimming filters, on the outer surface of which the CCD cell is attached. In order to attach it, in between the trimming filter and the CCD cell, there is inserted a spacer that is somewhat smaller than the cell and is provided with an air pocket for the radiation directed to the cell. The spacer keeps the CCD cell at the right distance from the color splitting prism, i.e. on the correct image plane, and it is made of a material that has the same thermal expansion coefficient as the prism, in order to maintain the cell alignment in place. Said spacer has rounded outer margins, and around the margins, the spacer is glued both to the CCD cell and to the trimming filter by means of external glue strips. Also in this arrangement, the problem is that the thermal expansion coefficient of the glue is different from the thermal expansion coefficient of the prism glass. Actually the fact that the spacer and the prism materials have the same thermal expansion coefficient does not remarkably reduce alignment errors, because said elements are heated at different speeds or remain at different temperatures.
From the publication JP-63-90985 there is known an alignment structure, where the CCD cells are attached on the exit surfaces of the color splitting prism by glue, by employing adjusting pieces between the prism and the element at opposite margins of the CCD cell. Each adjusting piece is glued to the CCD cell and to the prism exit surface by means of a glue that is hardened by ultraviolet light, in which case the cell can be moved, before the glue is hardened, with respect to the support elements, and the support elements can likewise be moved with respect to the prism. The purpose of the support elements is to reduce the deviation caused by the different thermal expansion coefficients of glue and glass in relation to the focusing and alignment, i.e. during the use of the apparatus.
Another problem related to the above described methods is that the glues shrink during the hardening process and consequently move the CCD cell, and thus the CCD cells are not reliably in their correct positions after the glue is hardened. On the other hand, while gluing a CCD cell directly onto the prism, the viscosity of the glue and its uneven distribution result in that the CCD cell is moved and must be aligned several times during the hardening process of the glue. Moreover, the heating of the CCD cell during use creates internal tensions in the glue layer, because the color splitting prism is not heated, or is heated at a different pace than the cell, which may cause the cell alignment to change, or even the element to fall off.
When employing above described ways to attach CCD cells, a common drawback is that heat is transferred out of the cell. Owing to the structure and properties of CCD cells, their noises as well as the basic signal levels are changed along with the temperature. The publication U.S. Pat. No. 5,596,228 discloses a cooling arrangement for a CCD cell, which arrangement comprises a thermally conductive cooling piece arranged against the rear surface of the cell, and a thermoelectric cooler, i.e. a so-called Peltier element, arranged against its outer surface. From the Peltier element, heat is conducted further away by means of a thermodynamic heat pipe based on Carnot cycle. The arrangement heats the CCD cell efficiently, but it is complicated in structure and therefore expensive, and consumes a lot of energy. Said publication does not mention that the CCD cell should be attached to an optical component, such as a prism or the like. On the other hand, it is mentioned that the CCD cell can replace film, for instance, which means that the cooler described in the specification is neither meant for a CCD line cell nor to be used together with a color splitting prism, but it is meant to be combined to a single CCD surface cell provided with a large number of element rows. When this type of a surface cell is meant for one-color, i.e. black and white imaging, each pixel therein is formed of one CCD element, and when the surface cell is meant for color imaging, each pixel is formed of three adjacent CCD elements, and in front of each CCD element, there is arranged one of the three color separation filters. These types of CCD surface cells are placed at a distance from the objective, so that the difference in temperature between the optical parts and the cell does not cause any problems. Also, the problem with alignment does not arise with these arrangements, because the electric signals corresponding to all three colors are created in one and the same CCD cell, in adjacent CCD elements.